of aging but some ageing no matter how slow will always occur. Mortality Mortality is the number of deaths, in a particular group, over a specific time period. There are two types of mortality: intrinsic and extrinsic mortality. Intrinsic mortality is defined as mortality due to ageing, the physiological decline due to innate processes, whereas extrinsic mortality is the result of environmental factors such as for example predation, starvation, accidents and others. Flying animals such as bats, for example, have fewer predators, and therefore have a low extrinsic mortality. Birds are warm-blooded and similar in size to many small mammals, yet often live 5–10 times as long. They face less predation than ground-dwelling mammals, and thus have lower extrinsic mortality. When examining the body-size vs. lifespan relationship, one also observes that predatory mammals tend to live longer than prey mammals in a controlled environment, such as a zoo or nature reserve. The explanation for the long lifespans of primates (such as humans, monkeys, and apes) relative to body size is that due to their intelligence, manage to achieve lower extrinsic mortality. Potential immortality of the germ line Individual organisms are ordinarily mortal; they age and die, while the germlines which connect successive generations are potentially immortal. The basis for this difference is a fundamental problem in biology. The Russian biologist and historian Zhores A. Medvedev considered that the accuracy of genome replicative and other synthetic systems alone cannot explain the immortality of germ lines. Rather Medvedev thought that known features of the biochemistry and genetics of sexual reproduction indicate the presence of unique information maintenance and restoration processes at the different stages of gametogenesis. In particular, Medvedev considered that the most important opportunities for information maintenance of germ cells are created by recombination during meiosis and DNA repair; he saw these as processes within the germ cells that were capable of restoring the integrity of DNA and chromosomes from the types of damage that cause irreversible ageing in somatic cells. Diseases Progeroid syndromes Progeroid syndromes are genetic diseases that are linked to premature aging. Progeroid syndromes are characterized by having features that resemble those of physiological aging such as hair loss and cardiovascular disease. Progeria Progeria is a single-gene genetic disease that cause acceleration of many or most symptoms of ageing during childhood. It affects about 1 in 4-8 million births. Those who have this disease are known for failure